1. Field of the Invention
This invention relates to imaging of objects by laser radar and more particularly to ranging or three-dimensionally imaging laser radar using high-gain semiconductor detectors.
2. Description of the Related Art
Active optical methods for imaging objects are known, including laser techniques often referred to as LADAR (Laser Detection and Ranging) or xe2x80x9cLIDARxe2x80x9d (Light Detection and Ranging). These methods are the optical analogues of the more well known microwave radar. In the optical techniques light (typically from a pulsed laser source) is directed toward an object of interest. Reflected light from the object is then typically gathered and focused to an image plane, where it falls on a photodetector. An array of photodetectors at the image plane can be used to detect a two-dimensional image frame, which can be further processed (often digitally) to enhance the image. If adequate light is gathered and the reflective emissions of the object are distinguishable from noise, an object can be identified by such a system.
Some LADAR systems also are capable of determining the distance to an object, for example by timing the travel time of light pulses sent round trip from a source to the object and back to a detector. However, adapting such systems to remote objects challenges the sensitivity and speed of available devices. The challenges are accentuated when a conspicuous light source is undesirable, as when the LADAR operator desires to remain covert.
In many laser imaging systems either the scanner or the receiver is scanned across the object, and multiple laser pulses are used to probe the contours of the object. Such systems are not particularly covert, as the multiple pulses can easily be sensed and the source thereby located.
Many of the known systems also suffer from other shortcomings. The photodetectors previously used have had relatively poor sensitivity and signal-to-noise ratio. Some require cryogenic cooling. Some offer poor ranging accuracy.
U.S. Pat. No. 5,892,575 to Marino (1999) discloses a system for imaging a scene using an array of monolithic light detectors operating in non-linear, Geiger mode. In one xe2x80x9cscannerlessxe2x80x9d embodiment disclosed by the patent, a processor develops an image of the target scene based on the round trip travel times of object-reflected photons received by a photodetector array at multiple positions on the array. The patented system addresses many of the shortcomings of the prior art, but it is nevertheless subject to certain drawbacks. Geiger-mode photodetector operation is subject to high levels of thermal noise, reducing the signal-to-noise ratio undesirably. Thus, the detector commonly requires cooling (although not necessarily to cryogenic temperatures). Moreover, Geiger mode hypesensitivity may cause false alarms due to scattered light from the atmosphere triggering a detector response.
The invention three-dimensionally images a subject scene by illuminating it with a laser pulse, then receiving reflected light from the scene using a focusing optical system and a pixellated array of photodetectors operating in a linear, non-geiger mode. The distances to the various surfaces in the scene are determined by measuring the relative times of arrival of the reflected photons at each pixel on the photodetector array. Based on the light detected and its time of arrival at each pixel, a three dimensional image is constructed and displayed. High sensitivity photodetectors operating in a linear, non-Geiger mode are preferably integrated with a high-gain, fast charge amplifier and a timing circuit at each pixel, enabling image acquisition with as few as one pulse.
A multi-pulse embodiment allows more accurate, xe2x80x9cvernierxe2x80x9d, measurement of the relative distance to scene features, using as few as two pulses. In another variation of the invention, the photodetectors in combination with the charge amplifier circuit can easily be switched to a passive mode, allowing a user to view the scene first without illumination, then switch to laser pulse illumination to acquire more scene information.
These and other features and advantages of the invention will be apparent to those skilled in the art from the following detailed description of preferred embodiments, taken together with the accompanying drawings, in which: